Various document feeders have been developed for feeding an accumulation of sheets or other documents. Typically, sheets of paper or any other documents such as checks or paper currency (hereinafter referred to as documents) are placed into a stack for feeding by the apparatus one at a time from the stack. The feeder separates the documents and feeds them at spaced intervals in single fashion, one at a time. The feeder can be a separate piece of equipment or part of a larger system such as an inserter system or a document folding system.
Various systems have been developed for determining the proper operation of the above noted type of feeders as well as for determining the proper operation of feeding equipment to detect the presence of two or more overlapping documents in the systems. This is often termed "double" detection.
The detection of doubles by a doubles detection system is required under various conditions and often for high speed feeding apparatus. In high speed feeding apparatus particularly, an improper feeding of two or more overlapped sheets or other documents can cause a jam in the system which can involve a large number of documents due to the speed of the system. Moreover, an improper feeding involving the overlapping of two or more documents, as for example in an inserter system, can result in improper collations of documents being assembled and thereafter inserted into the wrong envelope.
The detection of doubles is complicated in feeding equipment that is adapted to handle a wide variety of documents. This is because the documents can vary greatly in the thickness of the material, the print content, the graphics content thereon, as well as the color(s) of the documents as well as the color(s) of the printed content and graphics on such documents. Consider, the potential difficulty of detecting doubles, for example, for a two colored document with multicolored printing and graphics thereon and with a dark stripe along one side thereof. Moreover, the detection of doubles is further complicated by the presence of paper dust and other contaminants in sheet feeding equipment which can hinder proper operation of a doubles detection system.